Certain foods, beverages, and other packaged goods—such as beer and fruit juices, certain cosmetics and medicines, and the like—are sensitive to oxygen exposure, and require packages having high oxygen barrier to preserve the freshness of the contents and avoid changes in flavor, texture, or color. For many applications, the oxygen barrier properties of PET homopolymers and copolymers are satisfactory. However, for very oxygen-sensitive products, the oxygen barrier properties of such polymers do not provide adequate protection for the product.
A variety of approaches have been used to enhance the passive barrier properties of PET, including blends with high barrier polymers or additives that decrease the permeability of the resin, incorporation of impermeable fillers, the use of coated or multilayer structures, and copolymerization with comonomers that produce a lower permeability polymer than unmodified PET.
To further reduce the entry of oxygen into the contents of the package, oxygen-scavenging technologies have been developed for PET packages. These may include oxidizable moieties, such as polyamides, polydienes, or polyethers, blended or reacted into PET. Typically, small amounts of transition metal salts, such as cobalt salts of organic acids, are also incorporated to catalyze and actively promote the oxidation of the oxidizable moiety. The use of such oxidizable moieties, which chemically remove oxygen migrating through the walls of the package, can be a very effective method to reduce the oxygen transmission rates of plastics used in packaging.
U.S. Pat. No. 5,310,497 discloses a composition for scavenging oxygen that is said to have high oxygen scavenging rates at low temperatures. The composition comprises an ethylenically unsaturated hydrocarbon and a transition metal catalyst and can be incorporated into various types of layers. It is preferred that the composition be incorporated into layers of multilayered articles used for packaging oxygen-sensitive products such as food products.
U.S. Pat. No. 5,211,875 discloses a method of initiating oxygen scavenging by compositions that contain oxidizable organic compounds and transition metal catalysts. The method comprises initiating scavenging by exposing the composition to radiation. The method can be used for initiating scavenging in packaging layers or articles for oxygen sensitive products such as foods and beverages.
U.S. Pat. Nos. 5,021,515 and 5,955,527 disclose a wall for a package which comprises a polymer, and which is capable of scavenging oxygen through the metal-catalyzed oxidation of an oxidizable organic component. The oxidizable organic component may itself be a polymer, and preferred compositions are said to include a blend of 96% polyethylene terephthalate and 4% poly(m-xylyleneadipamide) containing 200 ppm cobalt as catalyst.
U.S. Pat. No. 6,083,585 discloses compositions for scavenging oxygen that comprise condensation copolymers comprising predominantly polyester segments and an oxygen-scavenging amount of polyolefin oligomer segments. The polyester segments comprise segments derived from typical bottling and packaging polyesters such as PET and PEN. The copolymers are preferably formed by transesterification during reactive extrusion and typically comprise about 0.5 to about 12 wt % of polyolefin oligomer segments. Use of these oxygen-scavenging compositions in bottles is said to provide a clear and rigid bottle similar in appearance to unmodified polyester bottles.
U.S. Pat. No. 6,544,611 discloses an oxygen-scavenging PET-based copolymer comprising from about 10 to about 120 ppm cobalt based on the PET polymer, and from about 15 to about 150 ppm zinc based on the PET polymer.
U.S. Pat. No. 6,863,988 discloses monolayer packages comprised of an oxygen scavenging composition suitable for direct contact with package contents and recycle with other polyester bottles are disclosed. The oxygen scavenging composition is comprised of a modified copolymer which is comprised of predominantly polyester segments and an oxygen scavenging amount of oxygen scavenging segments. The polyester segments comprise segments derived from typical bottling and packaging polyesters such as PET and PEN. Use of these oxygen scavenging copolymers in bottles provides a clear and rigid monolayer bottle similar in appearance to unmodified polyester bottles. In a series of preferred embodiments, bottles fabricated with the oxygen scavenging copolymers of this invention are over 99 wt % PET and contain less than 50 ppb of extractable components.
U.S. Pat. No. 7,186,464 discloses an oxygen barrier composition comprising an oxygen barrier polymer and an oxygen scavenging polymer. The composition can be in the form of a physical blend or a cross-linked blend, and can further comprise a compatibilizer, a transesterification catalyst, or both. Preferably, the oxygen barrier polymer is poly(ethylene/vinyl alcohol) (EVOH), polyethylene terephthalate (PET), or polyamide other than MXD6. Preferably, the oxygen scavenging polymer comprises an ethylenic backbone and a pendant cyclic olefinic group, or the oxygen scavenging polymer is a polyamide derived at least in part from a xylene diamine-based monomer. The oxygen barrier composition can be formed into an oxygen barrier layer of a packaging article. Such layers and articles, and methods for making same, are also disclosed.
U.S. application Ser. No. 11/364,916 discloses a composition comprising (i) an aromatic polyester resin, and (ii) a polydiene, where greater than 20 mole percent of the mer units of said polydiene have a 1,2 microstructure or the hydrogenated residue thereof.
U.S. patent application Ser. No. 11/495,431 filed Jul. 28, 2006 and having common assignee herewith, discloses polyester compositions that include aluminum atoms in an amount of at least 3 ppm, based on the weight of the polymer, and that further include alkaline earth metal atoms or alkali metal atoms or alkali compound residues, the polymers having an It.V. of at least 0.72 dL/g obtained through a melt-phase-only polymerization process (“melt-phase-only process”).
U.S. patent application Ser. No. 11/229,238, filed Sep. 16, 2005 and having common assignee herewith, discloses polyester compositions comprising polyester polymers, aluminum atoms, alkaline earth atoms or alkali metal atoms or alkali compound residues, and particles that improve the reheat rate of the compositions.
While compositions such as those described are effective oxygen scavengers, and find use according to the present invention, we have found that performance may vary significantly depending upon the nature of the polyester and the oxygen-scavenging component.
We have unexpectedly discovered that PET polymers made using the catalysts system of the present invention, when blended with an oxygen-scavenging polymer and a transition metal, result in polymer blends having better oxygen-scavenging properties than polymer blends made with PET polymers prepared using most conventional catalyst systems, while maintaining the properties that make the blends suitable for use in the packaging of oxygen-sensitive products, including transparency, miscibility, rigidity, good barrier properties, recyclability, and reasonable cost.